Aromatic compounds carbocations

Aromatic compounds such as benzene react with alkyl chlorides in Ihe presence of AlCl i catalyst to yield alkylbenzenes. The reaction occurs through a carbocation intermediate, formed by reaction of the alkyl chloride with AICI3 (R—Cl + A1CI 1 - U+ + AICl4 ). How can you explain the observaiion that reaction of benzene with 1-chloropropane yields isopropylbenzene as the major product ... 

Rearrangement is possible even with a non-carbocation mechanism. The rearrangement could occur before the attack on the ring takes place. It has been shown that treatment of CH3 CH2Br with AlBt3 in the absence of any aromatic compound... 

Antimony pentachloride is a reactive Lewis acid that can be used for Friedel-Crafts reactions and some other Lewis-acid-catalyzed reactions. The HF-SbF5 system is known as magic acid, and carbocations are stabilized in this medium.353 By using the HF-SbF5 system, alkylation of acetophenone (a relatively unreactive aromatic compound) has been achieved (Scheme 87). 

Whether you re confused by carbocations or baffled by biomolecules, this straightforward, easy-to-read guide demystifies Organic Chemistry II. From appreciating aromatic compounds to comprehending carbonyls, you ll discover what you need to know about organic reactions in order to master the course and score high on your exams ... 

Laali et al. have characterized carbocations generated from substituted polycyclic aromatic compounds. The related cation 157 is a true aryl-methyl-type ion, whereas cations 158 have arenium ion character because the strongly electron-withdrawing a-CF3 group enhances charge delocalization into the pyrenyl and phenyl groups. 

The fragmentation in the mass spectrometer of the molecular ion of aromatic compounds to produce benzylic carbocations was discussed in Section 15.6. Actually, the benzylic carbocation is thought to rearrange to an even more stable carbocation, also with m/z 91. that is the actual species that is detected. Suggest a structure for this carbocation and explain why it is so stable. 

A second limitation is that aromatic compounds substituted with moderately or strongly deactivating groups cannot be alkylated. The deactivated ring is just too poor a nucleophile to react with the unstable carbocation electrophile before other reactions occur that destroy it. 

The final limitation is one that plagues all carbocation reactions rearrangements. Because the aromatic compound is a weak nucleophile, the carbocation has a lifetime that is longer than is the case in most of the other reactions involving this intermediate, allowing ample time for rearrangements to occur. An example is provided by the following equation ... 

Despite these limitations, alkylation of readily available aromatic compounds, such as benzene and toluene, using carbocations that are not prone to rearrange, is a useful reaction. Intramolecular applications of this reaction have proven to be especially valuable. 

Carbocations, formation of oligomers and aromatic compounds Spectral signature of carbocations... 

Carbocations are perhaps the most important electrophiles capable of substituting onto aromatic rings, because this substitution forms a new carbon-carbon bond. Reactions of carbocations with aromatic compounds were first studied in 1877 by the French alkaloid chemist Charles Friedel and his American partner, James Crafts. In the presence of Lewis acid catalysts such as aluminum chloride (A1C13) or ferric chloride (FeCl3), alkyl halides were found to alkylate benzene to give alkylbenzenes. This useful reaction is called the Friedel-Crafts alkylation. 

Friedel-Crafts alkylation of aromatic compounds involves the formation of a carbocation that acts as electrophile (see section 2.1.3). 

The acyl halides (RCOX) on treatment with anhydrous aluminium chloride (AICI3) give a complex, which decomposes to give acyl electrophile, an acylium ion (RCO+). Friedel-Crafts acylation of aromatic compounds involves the formation of a carbocation that acts as an electrophile (see section 2.1.3). 

The interaction of certain electrophiles with an aromatic ring leads to substitution. These electrophilic reactions involve a carbocation intermediate that gives up a stable, positively charged species (usually a proton) to a base to regenerate the aromatic ring. Typical electrophiles include chlorine and bromine (activated by interaction with a Lewis acid for all but highly reactive aromatic compounds), nitronium ion, SO3, the complexes of acid halides and anhydrides with Lewis acids (see Example 4.5) or the cations formed when such complexes decompose (R— —O or Ar =0), and carbocations. 

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